Synthesis of monothiohydroxamic ligands and their lead complexes. Structures of N-methyl-3-pyridothiohydroxamic acid, bis(N-methyl-3-pyridothiohydroxamato) lead(II) and bis(N-cyclohexyl-phenylacetothiohydroxamato) lead(II)

An improved synthesis of thiohydroxamic acids is described. The reaction of carboxymethyl dithiophenylacetate with several substituted hydroxylamines in aqueous solution at neutral pH gives good yields of the resultant thiohydroxamic acids. The lead coordination chemistry of these compounds is being investigated as part of a program to develop lead-specific sequestering agents. The single crystal structure of bis(N-cyclohexyl-phenylacetothiohydroxamato) lead(II) shows that it is essentially trigonal bipyramidal, with the lead lone electron pair dominating the coordination geometry. The complex has crystallographic two-fold symmetry and the sulfur atoms occupy equatorial positions, with a PbO distance of 2.746 Å and PbO distance of 2.383 Å. The space group is P21212 with a = 14.146, b = 18.003, c = 5.373 Å, Z = 4. The structure of bis(N-methyl-3-pyridothiohydroxamato) lead(II) shows a similar geometry but a different isomer. The approximate trigonal bipyramidal geometry has the equatorial coordination positions occupied by one oxygen and a sulfur. The PbS distances are 2.725 (equatorial) and 2.809 (axial) Å. The corresponding PbO distances are 2.497 and 2.349 Å. The space group is P212121 with a = 7.512, b = 12.686, c = 21.583 Å, Z = 4. For comparison the crystal structure of the free ligand N-methyl-3-pyridothiohydroxamic acid has been determined. It is found in the E configuration (whereas the Z configuration is required for metal chelation). This represents the first such isomeric form for a thiohydroxamic acid, although both forms have been seen in hydroxamic acid crystal structures. The space groups is Pc, with a = 4.098, b = 8.999, c = 11.184 Å, β = 100.48°, Z = 2. The 207Pb NMR of a series of these complexes presents a convenient way to monitor the electronic environment of the lead nucleus; the resonances of these compounds occur in the range 1555-1493 ppm relative to PbMe4.